Vegetation and Wildlife
About 37 percent of the watershed is wooded. Dominant trees are Red Alder, Bigleaf Maple, Douglas-fir, Western Redcedar, and Western Hemlock. The forest understory in the Tryon Creek State Natural Area includes many trilliums; they are celebrated each spring during the park's Trillium Festival. More than 90 species of wildflower such as Fringecup are found in the park as well as plants such as Sword Fern. Invasive species found in many parts of the watershed include English Ivy, Himalayan Blackberry, English Holly, Garlic Mustard, and Western Clematis.
Riparian zones and floodplains are relatively intact along the lower reaches of the main stem in the state natural area, fairly intact in Marshall Park, and marginal in the upper reaches surrounded by homes. Arnold Creek has riparian corridors wider than 100 feet (30 m) in good condition, while Falling Creek's riparian zones, surrounded by homes, are in poor condition. Damaged zones from which native vegetation has been removed lead to erosion, stream bank failure, sedimentation, lack of shading, and higher stream temperatures.
Tryon Creek is among the few streams in the Portland metropolitan area with a run of Steelhead Trout, and Coho Salmon have been recorded spawning in the creek. Surveys by the Oregon Department of Fish and Wildlife in 2002 found Coho, Chinook Salmon, Steelhead, and Cutthroat Trout in different parts of the creek at different times of the year. None was present in large number. Cutthroat Trout had the biggest population, estimated at 53 individual fish during the spring of 2002. Water striders, invertebrates that can walk on water, are common in the pools of Tryon Creek. More than 60 species of birds, including Cooper's Hawks, Great Blue Herons, kingfishers, towhees, waxwings, and wrens frequent the area. Some of the resident mammals are bats, coyotes, moles, rabbits, skunks, and squirrels, while frogs, salamanders, snakes, and turtles also do well in the watershed.
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